Electronic components inside flat objects, such as tags, bank cards or ID cards require a low profile of typically less than 0.5 mm. It is anticipated that flat, low-cost, and low power real time clocks (RTCs) and frequency reference oscillators (RFOs) are required residing inside these flat objects for future applications in the e-security, pharmaceutical, and food industry. An oscillator comprises a resonator and a feedback amplifier circuit, which are connected in a closed feedback loop. State-of-the-art RTCs make use of quartz crystal resonators with a height profile that typically exceeds the allowed sub-mm specification needed for their incorporation into flat products like cards, tags, and sheets of (value) paper. The main reason for this is because the packaging technology being used to encapsulate the quartz crystal does not lend itself well to miniaturization.
Furthermore, quartz resonators cannot be integrated easily on a Si chip. Therefore, the integration of a complete oscillator consisting of the crystal and amplifier cannot be realized on a single chip and further prohibits the miniaturization of RTCs and RFOs. In contrast, a MEMS resonator can be processed and packaged using surface micro-machining techniques and can be integrated with the amplifier circuit to form a very small form-factor oscillator.
Surface micro-machining is a technique whereby freestanding and moveable structures are made on top of a substrate using thin film deposition and etching techniques. In this way, both the resonator and its package can be processed on top of e.g. a Si wafer. The packaged resonator has a height of only several thin films measuring about 10 μm in total thickness. Furthermore, surface micro-machining allows for the definition of many thousands of packaged resonators onto a single wafer without making use of expensive assembly steps. The production cost associated with micro-machining decreases when the area occupied by a single device decreases. In this way, miniaturization of the resonator also has cost advantages. For quartz resonators, the production cost increases when the size of the resonator decreases as a result of the assembly-like production process that is being used.
MEMS-resonator based oscillators thus allow for low profile and low cost clocks and oscillators. However, they do not necessarily consume little power. Piezoresistive MEMS resonators require a body DC bias-current & a DC polarization voltage for their electrode. The body of a piezoresistive resonator is fed a DC-current. By applying an AC-signal to an attached but isolated electrode (‘gate’) the resistivity of the body is modulated, so a signal-voltage develops. This takes place in a narrow frequency-region for proper operation.
Parasitic capacitances resulting from the physical layout cause undesired conduction paths through the structure, for example a feedthrough path from the drive electrodes to the sense electrodes. These limit the performance of the oscillator and increase the power consumption.
Low power consumption and high performance are often conflicting requirements: to obtain amplitude selectivity that exceeds the signal transfer caused by the inherent capacitive feedthrough path, more bias and/or polarization are required. This increases power consumption. Power consumption can be lowered and/or amplitude selectivity can be increased if the capacitive feedthrough path can be eliminated or at least be reduced.